1. Field of the Invention
The invention relates to energy control for excimer and molecular fluorine gas lasers, and particularly to control and feedback software algorithms and gas replenishment for maintaining constant laser output emission pulse energies and/or application process energy doses.
2. Discussion of the Related Art
The energy of output emission pulses of an excimer or molecular fluorine laser will decrease continuously unless certain input parameters or conditions are controlled during the operation of the laser. This is due to halogen consumption by reactions of the halogen gas within the gas vessel and halogen burn up by the gas discharge. Additionally, the output power will decrease due to build up of gas contamination.
An excimer laser can be operated for a certain time at a constant energy level if the charging voltage is continuously increased to compensate these factors which cause energy losses. For demanding applications like lithography or TFT annealing, it is desired to maintain control of the charging voltage, as well as other beam parameters, in addition to pulse energy or laser output power. Therefore, more sophisticated processes were developed. When a maximum applicable charging voltage is reached, gas replenishment actions can be performed to further extend the operation time at constant energy of the laser. Such gas replenishment actions may be performed to compensate halogen depletion and for contamination reduction. Halogen depletion is typically compensated by halogen injections (HI). Contamination reduction is achieved by partial gas replacements (PGR).
Gas replenishment was introduced around 1986 for excimer lasers (see U.S. Pat. No. 4,997,573, which is hereby incorporated by reference). Gas replenishment actions may be triggered when the charging voltage exceeds a preset level. Gas replenishment actions have been characterized in the past by significant reductions in charging voltage. Large variations in charging voltage during long constant energy operation periods are a disadvantage, however, because such large variations in charging voltage can affect various beam parameters other than beam energy or power. In other words, large variations in charging voltage for stabilizing the output energy serve to destabilize other important beam parameters.
In order to maintain both the charging voltage and the output energy or power of the laser at substantially constant levels, large gas replenishment actions were replaced by smaller gas actions such as micro halogen injections (HI) and micro partial gas replacements or mini gas replacements (GR or mGR) (see U.S. patent application Ser. No. 09/447,882 and No. 60/171,717, each of which is assigned to the same assignee and is hereby incorporated by reference). These micro or mini gas replenishment actions preferably result in little or no disturbance in charging voltage that is detectable with sufficient precision under industrial operation conditions. Therefore, it is desired to use another parameter other than changes in the charging voltage to trigger the micro or mini gas replenishment actions. One parameter that may be used is the number of laser pulses, or pulse count, as a suitable trigger for micro or mini gas replenishment actions. This was disclosed in U.S. Pat. No. 5,097,291 and later in U.S. Pat. No. 5,337,215, each of which is hereby incorporated by reference. For example, a gas replenishment action may be performed periodically approximately every 100,000 pulses.
For microlithography scanner systems, it is desired to maintain constant energy dose when scanning over a die site on a wafer. The scanning speed, the exposure slit width and the laser repetition rate determine the number of pulses overlaid at each site on the wafer. The number of overlaid pulses is dependent on the application process. For example, approximately 40 pulses may be overlaid at a die site, whereas a typical length of a burst may be between 100 and 500 pulses.
The constant energy dose for each site on a wafer may be specified by a moving energy average. Precise dose control may then be observed as low fluctuation in moving energy average. The output energy of the laser may be controlled by changing the high voltage (HV) that is used for a discharge in the laser tube. The output energy can be and typically is measured for each pulse, and also the HV can be changed for each individual pulse.
Excimer and molecular fluorine lasers may be typically operated in burst mode. This means that the laser generates a xe2x80x9cburstxe2x80x9d of pulses, such as 100 to 500 pulses as mentioned above at a constant repetition rate, followed by a burst break or pause of from a few milliseconds up to a few seconds while the stepper/scanner does some wafer positioning. A burst break may be a short burst break such as may occur when the beam spot is moved to a different location on a same wafer, or may be a long burst break such as would occur when the stepper/scanner changes the wafer.
When an excimer or molecular fluorine laser is operated in burst mode, the first few pulses of each burst will have a higher pulse energy than later pulses if left uncompensated. Therefore, the moving average at the beginning of a burst would be higher than later in the burst. It is desired to compensate this overshoot in order to achieve a constant energy dose. Overshoot compensation may be achieved by reducing the charging voltage for the first few pulses (see U.S. Pat. Nos. 5,463,650, 5,710,787 and 6,084,897, each of which is hereby incorporated by reference). This allows the energy dose to be kept constant at the beginning of a burst sequence. The charging voltage is adjusted for each laser pulse at the beginning of a burst below that which is applied to pulses later in the burst.
The problem that the first few pulses after a burst break (at the beginning of a burst) have a higher ratio of energy to HV than the pulses in the middle or at the end of a burst, can be understood by observing what happens when the HV is kept constant during the burst, as illustrated in the sketch of FIG. 1. In the sketch of FIG. 1, the first 5 to 10 pulses have a high energy, and then the energy first decays rapidly, and then more slowly until after 20 to 100 pulses the energy reaches a constant level. This phenomenon is called overshoot or spiking.
In order to keep the pulse energy or energy dose constant (which is desired during laser operation), one uses a low HV for the first pulses of a burst and then increases the HV to a constant level during the burst. This is done in response to the overshoot of the pulse energy that will otherwise occur as just described.
The exact behavior of the energy is affected by various parameters in a way that is difficult to predict. It is desired to have a technique for predicting the HV for the next pulse so that the energy of the next pulse or the energy dose at the application process will meet the target energy or target energy dose.
There are short-term effects and long-term effects that influence the behavior associated with the energies of pulses during burst and from burst to burst. Short-term effects may last for only a few seconds or less. Long term effects include gas aging (several days), tube aging (several months) and maybe optical effects (years). These effects may be taken into account by changing controller parameters. The parameter adaptation may be advantageously performed automatically.
The energy behavior changes, depending on the length of the burst break, the repetition rate of the laser, the energies of the most recent pulses and other effects. It is more difficult to control the energies of the first pulses in a burst than it is to keep the energy or energy-dose constant for pulses at the middle and end of a burst because gas conditions do not change as rapidly with time over the duration of the burst. It is thus desired to have pulse energy or energy dose control algorithm that produces high pulse energy or energy dose stability at the beginning of a burst, and also throughout the entirety of the burst.
Gas aging depends on time and input energy into the electrical discharge. A typical time constant for gas replenishment actions based on time may be several hours, e.g., eight hours. Depending on the laser wavelength and the particular laser construction and operation mode and other parameters, the time constant can be as low as 1 hour or less, and it can be more than a day.
State of the art gas replenishment is based on pulse count (see, e.g., the ""241 and ""215 patents, mentioned above). A gas replenishment algorithm based on pulse count would work very well in the case of constant energy input into the laser discharge for each laser pulse or group of pulses. It is recognized in the invention that the input energy into the laser discharge is not, however, constant for each laser pulse or group of pulses over periods of laser operation. Particularly in industrial lithography processes, the input energy into the discharge is not constant for each laser pulse or group of pulses, particularly over many thousands of pulses. It is desired then to have a gas replenishment algorithm for performing gas replenishment actions based on those parameters such as time and input energy to the electrical discharge, upon which gas aging more closely depends, rather than pulse count.
It is therefore an object of the present invention to provide a gas replenishment algorithm that is based on parameters upon which gas aging more closely depends than pulse count, such as input energy to the electrical discharge, and also preferably time.
It is a further object of the invention to provide a pulse energy or energy dose control algorithm that produces high pulse energy stability or energy dose stability for initial pulses of a burst, and also preferably throughout the entirety of the burst.
In a first aspect of the invention according to the above objects, a gas discharge laser system is provided which includes a discharge chamber having multiple electrodes therein and containing a gas mixture including one or more components which are subject to depletion, a power supply circuit coupled to the electrodes for energizing the gas mixture, and a resonator for generating a laser beam. A processor monitors the accumulated energy applied to the discharge of the laser as a measure of gas mixture status, and a gas control unit serves to replenish the gas mixture based on the monitored accumulated energy applied to the discharge. The processor preferably also monitors the time as an additional measure of gas mixture status, and the gas control unit replenishes the gas mixture based on the monitored time in addition to the accumulated energy applied to the discharge. Charging voltage and variations of charging voltage may also be monitored along with the accumulated energy applied to the discharge and/or the time, and the gas control unit replenishes the gas mixture based on the monitored charging voltage and/or variations in the charging voltage in addition to the accumulated energy to the discharge and/or the time.
A method of stabilizing during operation a gas mixture initially provided within a discharge chamber of a gas discharge laser with an initial composition including one or more component gases that are subject to depletion is also provided. The method includes the steps of monitoring accumulated energy applied to the discharge of the laser, and determining the status of and/or adjusting the gas mixture based on the monitored accumulated energy applied to the discharge. Time is also preferably monitored along with the accumulated energy applied to the discharge. Charging voltage and variations of charging voltage may also be monitored along with the accumulated energy applied to the discharge and/or the time.
Therefore, in accord with a first aspect of the invention, it is of advantage to trigger gas replenishment actions on accumulated energy applied to the discharge. The new method is more flexible and therefore more powerful than, e.g., triggering gas replenishment actions on pulse count. If the pulse energy is varied then the new algorithm advantageously extends or shortens the gas replenishment interval correspondingly, thereby improving the laser performance.
In a second aspect of the invention according to the above objects, a method is provided for controlling output energies of successive pulses from a gas discharge laser operating in burst mode and characterized by emitting bursts of several pulses each followed by one of a long burst break and a short burst break depending on specifications of an application process. The method includes the steps of measuring the energies of at least a predetermined number of initial pulses of a first burst occurring after a long burst break, calculating values of input voltages for each of the initial pulses that would bring corresponding output energies of the initial pulses to a substantially same predetermined value for a subsequent first burst following a similar long burst break, and applying input voltages corresponding to the calculated values in a subsequent first burst after a similar long burst break such that pulses generated thereby have the substantially same predetermined output energy value.
The method may further include measuring the energies of at least a predetermined number of initial pulses of at least one second burst occurring a short burst break after a first burst following a long burst break, calculating values of input voltages for each of the initial pulses of the second burst that would bring corresponding output energies of the initial pulses of the second burst to a substantially same predetermined value for a subsequent second burst following a similar short burst break after a first burst following a similar long burst break, and applying input voltages corresponding to the calculated values for the initial values in the subsequent second burst following said similar short burst break after said first burst after said similar long burst break such that pulses generated thereby have said substantially same predetermined output energy value.
The method may further include measuring the energies of at least a predetermined number of initial pulses of at least one third or later burst occurring at least two short burst breaks after a long burst break, calculating values of input voltages for each of the initial pulses of the third burst that would bring corresponding output energies of the initial pulses of the third burst to a substantially same predetermined value for one or more subsequent bursts which occur after at least two short burst breaks following a long burst break, and applying input voltages corresponding to the calculated values for the one or more subsequent bursts after the two short burst breaks following the long burst break such that pulses generated thereby each have a substantially same predetermined output energy value.
The method may further include repeatedly applying the input voltages corresponding to the calculated values for the subsequent burst after the two short burst breaks following the long burst break to generate thereby additional bursts with initial pulses each having the substantially same predetermined value. The method may also further include measuring output laser energies corresponding to later pulses in a burst in addition to the initial pulses, calculating values of input voltages corresponding to each of these later pulses that would bring output energy doses of the laser, corresponding to sums of pulse energies of consecutive pulses, each to a substantially same predetermined energy dose value, and applying input voltages corresponding to the calculated values for bringing output energy doses to the substantially same predetermined value, such that energy doses associated with groups of pulses generated thereby each have a substantially same predetermined energy dose value.
Further in accord with the second aspect of the invention, an energy control software algorithm is provided for controlling output energies of successive pulses in a burst of pulses from a gas discharge laser operating in burst mode and characterized by emitting bursts of several pulses each followed by one of a long burst break and a short burst break depending on specifications of an application process. The algorithm provides a first table of input voltage values to be read by a processor which signals a power supply circuit to apply voltages according to the voltage values in the first table to thereby generate initial pulses in a subsequent first burst of output laser pulses after a long burst break each having a substantially same energy value. The input voltage values in said first table are calculated from measured data of initial pulses in a previous first burst after a long burst break. The input voltage values are used for producing the initial pulses each at a substantially same energy value.
The algorithm preferably further provides in a similar manner a second table of input voltage values to be read by the processor which signals the power supply circuit to apply voltages according to the voltage values in the second table to thereby generate initial pulses in a subsequent second burst of output laser pulses occurring after a short burst break following a first burst after a long burst break. The algorithm further provides in a similar manner a third table of input voltage values to be read by the processor which signals the power supply circuit to apply voltages according to the voltage values in the third table to thereby generate initial pulses in a subsequent third or later burst occurring after a short burst break following first and second bursts after a long burst break.
The table is created according to the steps of measuring energies of initial pulses of a first burst following a long burst break, calculating values of input voltages for initial pulses based on the measured initial pulse energies that would bring output energies of the laser corresponding to each of the initial pulses to a substantially same predetermined value for a subsequent first burst following a similar long burst break, and storing the calculated values of input voltages for initial pulses as the table, such that pulses generated according to input voltage values stored in the first table have a substantially same predetermined output energy value.
The algorithm preferably further provides that the table is updated according to the further steps of measuring energies of initial pulses of a subsequent first burst following a subsequent long burst break, calculating values of input voltages for initial pulses based on the measured initial pulse energies of the subsequent first burst that would bring output energies of the laser corresponding to each of the initial pulses to a substantially same predetermined value for another subsequent first burst following another subsequent long burst break, and updating the values in the table of input voltages for initial pulses in the first table using the calculated values of input voltages for initial pulses based on the measured initial pulse energies of the subsequent first burst, such that pulses generated according to input voltage values stored in the first table have a substantially same predetermined output energy value.
Preferably, in the second aspect of the invention, the energy dose of groups of pulses after the initial pulses, or of all groups of pulses is kept constant. This means that the sum of n pulse energies, e.g., for n=40 pulse energies, is kept constant for each package of n subsequent pulses. This sum, divided by the number of pulses in it, is referred to as the moving average.